The traditional reliance on fullerene-based ETLs in tin perovskite solar cells hinges on their ability to facilitate efficient electron extraction and transport, effectively boosting the cell’s overall power conversion efficiency. Despite these benefits, fullerenes are plagued by a range of issues that have slowed the commercial viability of tin-based perovskites. Their synthesis is laborious and expensive, involving several purification steps that raise production costs. More critically, the intrinsic electron mobility of fullerene materials is often insufficient to meet the demands of rapidly developing large-area solar modules. Furthermore, fullerenes interact with the perovskite absorbers in a manner that can cause spatial non-uniformities, ultimately limiting device reliability and lifespan. These limitations have inspired a rigorous search for materials that can emulate fullerene’s conductive properties without inheriting their drawbacks.

Enter the novel class of fluorinated triple-acceptor polymers, designed specifically to address these multifaceted challenges. The polymers, referred to as P1, P2, and P3 in the study, represent a new paradigm in ETL design. Unlike the rigid, spherical fullerenes, these polymers offer tremendous structural flexibility, enabling the formation of continuous and conformal interfaces with tin perovskite layers. This intimate interfacing is critical, as it ensures efficient electron extraction and suppresses non-radiative recombination pathways, which are detrimental to overall device performance. Remarkably, these polymers possess higher intrinsic electron mobilities, a property that dramatically enhances charge transport dynamics within the device architecture.

Among the polymers investigated, P3 stands out due to its optimal energy-level alignment with the corresponding tin perovskite absorbers. This alignment is crucial because it facilitates more efficient electron transfer processes, minimizing energy losses that typically hamper solar cell performance. The study reports that solar devices utilizing P3 as the ETL achieved power conversion efficiencies of 16.06% on small-area cells (0.04 cm²) and 14.67% on larger-area (1 cm²) cells. Notably, these figures have been independently certified at 15.90% and 14.51%, respectively, showcasing reliable performance metrics that are competitive with or even surpass those obtained using traditional fullerene-based layers.

The move away from fullerene architectures toward polymeric ETLs like P3 is more than a marginal improvement; it fundamentally changes the scalability landscape of tin perovskite solar cells. One of the enduring hurdles in perovskite research has been the challenge of uniformly covering large substrates without sacrificing device efficiency or longevity. Polymer ETLs inherently facilitate better morphological control, enabling the formation of uniform layers across centimeter-scale substrates. This capacity for scaling up holds the promise to bridge the gap between laboratory-scale experimentation and industrial-level production, bringing tin-based photovoltaics closer to widespread commercialization.

A critical aspect of solar cell viability beyond initial efficiency is long-term operational stability. Tin-based perovskites, in particular, have been susceptible to degradation triggered by ambient moisture, oxygen, and continuous illumination. The new polymers incorporate long-alkyl side chains and fluorine substituents, imparting significant hydrophobicity to the ETL surface. This hydrophobic nature effectively repels moisture infiltration while simultaneously mitigating chemical degradation pathways. As a result, devices featuring P3 retained over 85% of their initial efficiency even after 550 hours of continuous exposure to 1-sun illumination—a remarkable demonstration of durability that elevates the material’s prospects for real-world applications.

Beyond performance and stability, the cost and environmental impact of materials remain pivotal considerations in next-generation solar technology development. The fluorinated triple-acceptor polymers introduced here benefit from comparatively straightforward synthesis routes relative to the complex, multistep processes associated with fullerene preparation. This simplicity translates into reduced production costs and less environmental waste, offering a sustainable pathway to scale up manufacturing without compromising device function. These attributes make the polymers especially attractive for tackling one of the most pressing industry demands: affordable and eco-friendly solar solutions.

The implications of employing non-fullerene ETLs ripple through various facets of materials science and device engineering. For instance, the strong and uniform interaction between polymer ETLs and tin perovskite layers helps suppress defects and trap states at the interface, phenomena notorious for hampering efficiency and accelerating degradation. Additionally, this strong interfacial coupling enables greater control over electronic properties, which could lead to further fine-tuning of device architectures and the integration of complementary functionalities such as tandem cell stacking or flexible substrates.

Researchers also underscore the potential versatility of these polymers beyond just tin-based perovskites. Given their tunable chemical structures and energy levels, similar fluorinated triple-acceptor polymers might be adapted to interface effectively with lead-halide perovskites or other emerging photovoltaic materials. This adaptability could broaden the scope of high-performance, stable, and scalable solar cells accessible through this innovative material platform.

The study’s achievement of certified efficiencies at practical device sizes is particularly noteworthy in light of historical challenges in tin perovskite research. While lead-based perovskites have dominated efficiency records, tin alternatives have lagged due to intrinsic instability and suboptimal interfaces. This research knocks down one of the principal barriers—efficient electron transport and interface engineering—demonstrating that tin perovskites can rival their lead counterparts in both efficiency and stability when paired with the right transport layers.

In conclusion, the pioneering work presented in this study elucidates a clear pathway for the evolution of tin-based perovskite photovoltaics. Through the elegant design and implementation of fluorinated triple-acceptor polymer ETLs, these devices achieve superior electron transport properties, better energy alignment, enhanced durability, and scalable fabrication. The breakthrough lays an important foundation for ongoing efforts to develop cost-effective, environmentally friendly, and commercially viable solar energy solutions employing non-toxic tin perovskites. It also signals a critical shift in material design philosophy, highlighting the power of polymer chemistry to overcome longstanding limitations in perovskite solar cell technology.

As the photovoltaic research community digests these findings, future investigations will likely focus on optimizing polymer synthesis, exploring new polymer-family analogs, and integrating these ETLs into advanced device architectures. The promising balance of performance, stability, and manufacturability exemplified by P3 and its counterparts could redefine how tin perovskite solar cells are conceived, manufactured, and deployed. Such material innovations are poised to accelerate the global transition toward sustainable energy with affordable, efficient, and scalable solar technologies.

Ultimately, this work marks a pivotal contribution to the field of next-generation photovoltaics, bridging gaps between academic innovation and industrial application. It not only expands the material toolbox available for energy conversion technologies but also charts a future where fullerene-free, polymer-based ETLs become standard bearers for high-performance tin perovskite solar cells. Through this advancement, the scientific community edges closer to unlocking the full potential of perovskite materials as a cornerstone of renewable energy infrastructure worldwide.

Subject of Research: Advancement of non-fullerene electron transport layers in tin-based perovskite solar cells to improve efficiency, stability, and scalability.

Article Title: Centimetre-scale fullerene-free tin-based perovskite solar cells with a 14.51% certified efficiency.

Article References:
Li, T., He, F., Shen, T. et al. Centimetre-scale fullerene-free tin-based perovskite solar cells with a 14.51% certified efficiency. Nat Energy (2025). https://doi.org/10.1038/s41560-025-01919-1

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41560-025-01919-1

Electron-transport layers crafted with SnO₂ via CBD serve as critical mediators in efficiently extracting and transporting electrons generated in the perovskite absorber layer. Central to achieving high device performance is the formation of ETLs exhibiting uniform morphologies, minimal defect densities, and robust interfacial properties. The conventional CBD synthesis of SnO₂ typically proceeds via two competing nucleation pathways: cluster-by-cluster aggregation and ion-by-ion growth. Unfortunately, the cluster-by-cluster pathway often dominates, leading to heterogeneous deposition characterized by incomplete surface coverage and the formation of defects detrimental to charge transport and recombination dynamics.

The novel approach employs an excess ligand environment that fundamentally alters the deposition kinetics, effectively suppressing the cluster-by-cluster mechanism while preferentially facilitating the ion-by-ion growth pathway. This strategic modulation of growth dynamics enables the formation of highly uniform, conformal SnO₂ thin films, even on large substrates, within markedly reduced synthesis times. The resulting films exhibit substantially suppressed defect densities, manifesting superior optoelectronic properties necessary for high-performance solar devices.

A pivotal metric reflecting the quality of these SnO₂ films is the surface recombination velocity, which is critically lowered to 5.5 cm s⁻¹ in this work. The surface recombination velocity quantifies the propensity for photogenerated carriers to recombine at interfaces before contributing to photocurrent. Lowering this parameter illustrates the efficacy of the excess ligand strategy in passivating surface states and minimizing trap-assisted recombination, an achievement rarely realized with conventional CBD films.

Moreover, the luminescent properties of these SnO₂ films set new benchmarks in the realm of perovskite photovoltaics. Exhibiting an electroluminescence efficiency of 24.8%, these ETLs contribute not only to improved charge extraction but also function effectively as recombination suppression layers, which is vital for device stability and long-term operational performance. This high electroluminescence yield reflects a pronounced reduction of non-radiative recombination pathways, underscoring the quality of the interface engineered via this method.

The impact of these advanced SnO₂ ETLs permeates through the entire solar cell architecture, culminating in impressive photovoltaic efficiencies. The study reports a power-conversion efficiency (PCE) of 26.4% for champion perovskite solar cells, which aligns with or surpasses state-of-the-art figures. Such efficiencies herald the maturity of the excess ligand CBD method as a scalable technique capable of supporting commercial-level device fabrication.

Extending beyond individual cells, the practical scalability of this technique is demonstrated by achieving a 23% efficiency in perovskite solar modules, devices that integrate multiple cells to deliver higher voltage and practical power outputs suitable for real-world applications. Furthermore, the method’s compatibility with carbon-based perovskite cells yielding an efficiency of 23.1% showcases its versatility and adaptability to diverse device architectures, broadening the potential market impact.

Technically, the suppression of the cluster aggregation path involves the deliberate introduction of ligand molecules in excess relative to conventional protocols. These ligands coordinate with tin ions, stabilizing them and moderating nucleation kinetics. This molecular-level control steers the growth preferentially towards direct ion-by-ion deposition onto the substrate, circumventing the formation of colloidal SnO₂ clusters that compromise film integrity.

Such ligand-rich environments also provide biochemical passivation of surface defects, which act as recombination centers in the final film. By saturating these sites during growth, the excess ligand method mitigates mid-gap states and traps that traditionally plague ETLs derived from wet chemical methods. Consequently, charge carriers experience smoother transit through the ETL, enhancing open-circuit voltage and fill factor metrics in the integrated devices.

The rapidity of this deposition approach marks an additional industrial advantage. Conventional CBD techniques require extended durations to achieve coverage uniformity, a major bottleneck when moving toward manufacturing scale. By shifting the reaction pathway kinetics, the excess ligand strategy compresses processing times without sacrificing film quality or uniformity. This reduction in synthesis time translates directly to cost savings and higher throughput in manufacturing environments.

Importantly, the framework of this study also tackles oxidation-related degradation issues. In traditional solution-processed SnO₂ films, uncontrolled oxidative growth can induce variable stoichiometries and localized defects. The controlled ligand environment buffers the chemical milieu, leading to stoichiometrically consistent, phase-pure SnO₂ layers with improved chemical robustness, an essential factor for device longevity.

This research not only bridges the gap between laboratory-scale device fabrication and industrially feasible production but also enhances our fundamental understanding of nucleation dynamics in chemical bath depositions. The manipulation of ligand content as a lever to control nucleation pathways opens avenues for similarly structured approaches in other oxide semiconductors, potentially revolutionizing the fabrication of electron-transport layers beyond SnO₂.

The implications extend into the broader context of perovskite photovoltaics, where enhancing interface quality is crucial for overcoming stability and efficiency bottlenecks. Defect suppression at the ETL/perovskite interface reduces hysteresis phenomena and photodegradation pathways, two persistent challenges inhibiting broader adoption of perovskite solar technology. By addressing these through material synthesis innovation, the study brings the community closer to realizing commercially viable perovskite modules.

Looking forward, integrating the excess ligand CBD method with roll-to-roll processing and other scalable deposition techniques posits a promising route toward flexible, lightweight solar modules with low manufacturing costs. The combination of superior performance metrics and scalable fabrication processes could accelerate the deployment of perovskite-based photovoltaics in large-scale energy projects.

In conclusion, the discovery of an excess ligand strategy in the chemical bath deposition of SnO₂ represents a paradigm shift in fabricating electron-transport layers for perovskite solar cells. By tailoring nucleation pathways to prioritize ion-by-ion growth over cluster aggregation, researchers have achieved uniform, defect-minimized films with exceptional optoelectronic attributes. This advancement translates directly into record-setting device efficiencies and scalable production capabilities, fostering new possibilities in the sustainable energy industry. As the photovoltaic sector intensifies its quest for superior materials and processes, this work stands poised to inspire future innovations that bridge the divide between research breakthroughs and real-world applications.

Subject of Research: Electron-transport layers in perovskite solar cells; chemical bath deposition of tin oxide.

Article Title: Efficient and luminescent perovskite solar cells using defect-suppressed SnO₂ via excess ligand strategy.

Article References:
Seo, G., Yoo, J.J., Nam, S. et al. Efficient and luminescent perovskite solar cells using defect-suppressed SnO₂ via excess ligand strategy. Nat Energy (2025). https://doi.org/10.1038/s41560-025-01781-1

Image Credits: AI Generated

At the core of this research is the newly developed technique called Ultrafast Dark-field Momentum Microscopy. This cutting-edge approach enables scientists to investigate the fast dynamics of dark excitons with unprecedented temporal resolution. Unlike traditional excitons that emit light, dark excitons are a unique pair consisting of an electron and the hole left behind when the electron is excited. Their intriguing behavior has been a frontier in physics, remaining mostly undetectable until now due to the inability to visualize them directly. The research team, working under the guidance of Professor Stefan Mathias, has devised this method specifically to monitor how these fundamental particles are formed and behave in real-time within two-dimensional materials.

The ability to measure the dynamics of dark excitons is a significant leap forward. One of the challenges researchers face in materials science is detecting energy carriers that don’t naturally emit light. Dark excitons are exactly that; they possess energy but remain invisible in optical experiments. Traditionally, their dynamics were theorized, but now, with the capabilities of Ultrafast Dark-field Momentum Microscopy, researchers can provide solid empirical evidence of their existence, formation, and behavior. The study showcases how these particles are created within a matrix of tungsten diselenide (WSe₂) and molybdenum disulphide (MoS₂) in a staggering time frame of just 55 femtoseconds—a duration that is difficult to comprehend but incredibly significant in the realm of quantum mechanics.

This technique’s precision, noted by Dr. David Schmitt, the lead author of the study, provides invaluable insights into how dark excitons interact with their environment. The resolution of this research, measured at 480 nanometres, allows scientists to understand the intricate dynamics at the atomic scale. Such precise measurements can significantly impact the way we approach the development of new materials, particularly those intended for energy conversion and storage. With the enhanced understanding of how dark excitons operate, there lies the potential for improving the efficiency and quality of solar cells, presenting a promising pathway to harness solar energy more effectively.

Additionally, the significance of this research extends beyond just solar cells. The ability to observe and manipulate dark excitons can lead to advancements in a range of technologies focused on light emission and detection. For instance, innovations in LED technology and photodetectors might arise from a deeper understanding of dark excitons. These developments could foster better performance in technologies that rely on the conversion of light into energy, thereby expanding the frontiers of energy-efficient devices.

The research also highlights how dark excitons act as critical carriers of energy within two-dimensional materials. The Coulomb interaction allows these particles to maintain a connection even when the electron has effectively “flown away,” creating new opportunities for manipulating and utilizing energy within a semiconductor lattice at an atomic level. Understanding this interaction is a key aspect for scientists and engineers aiming to design future materials with optimized properties for specific applications in electronics and photonics.

This research was supported by substantial funding from the German Research Foundation (DFG), through several collaborative research centers dedicated to exploring energy conversion at atomic scales. Such support underscores the importance of this work in advancing fundamental sciences, ultimately translating to applied technologies that hold the potential for transformative impacts across multiple sectors, including renewable energy.

In conclusion, this pioneering study offers a substantial leap in our understanding of dark excitons and demonstrates the immense potential of Ultrafast Dark-field Momentum Microscopy to revolutionize how we perceive and manipulate energy carriers within materials. As we venture further into the nanoworld of photonics and semiconductor physics, groundwork laid by this research could facilitate significant advancements in modern energy technologies, offering promising solutions in our ongoing quest for efficient and sustainable energy sources.

The implications of this research resonate far beyond theoretical advancements; they provide tangible pathways to practical applications that can benefit society at large. With ongoing exploration and innovation in this field, researchers and engineers may soon unlock even more about the fundamental nature of excitons and their role in future technologies.

Subject of Research: Dark excitons in two-dimensional materials
Article Title: Ultrafast nano-imaging of dark excitons
News Publication Date: 3-Jan-2025
Web References: https://doi.org/10.1038/s41566-024-01568-y
References: David Schmitt et al. Ultrafast nano-imaging of dark excitons. Nature Photonics (2025). DOI: 10.1038/s41566-024-01568-y
Image Credits: Credit: Lukas Kroll

Keywords

Photovoltaics, Quantum dynamics, Ultrafast microscopy, Energy conversion, Dark excitons, Semiconductor physics, Two-dimensional materials, Photonics, Optoelectronics, Solar energy, Electrons, Atomic scale dynamics.